The optimum angles at which the inlet and exhaust valves of an internal combustion engine should open and close, both in relation to one another and in relation to the engine crankshaft, vary with the engine speed and load conditions. In an engine with a fixed valve timing, a compromise setting must be adopted in which different performance parameters are traded off one against the other.
To achieve improved performance over a range of engine speeds and loads, it has already been proposed to use variable phase couplings to vary the phase of a camshaft in relation to the crankshaft and in relation to another camshaft.
Several variable phase couplings are known from the prior art, each having its own advantages and disadvantages. Noise and wear are particularly serious common problems that are caused by the fact that camshafts are subjected to torque reversal during operation. While a valve is being opened by a cam on the camshaft, torque has to be applied to the camshaft in one direction to overcome the resistance of the valve spring. On the other hand, while a valve is closing, its spring attempts to accelerate the camshaft and the camshaft experiences a torque reaction from the valve train acting in the opposite direction.
A further problem with some known designs is that they cannot be retro-fitted to an existing engine because they require major modification to the engine block, cylinder head or valve train.
EP-A-0723094, which is believed to represent the closest prior art to the present invention, discloses a variable phase coupling for adjusting the phase between first and second rotatable members that addresses many of the above problems. The coupling comprises a first rotatable member within which there is coaxially mounted a second rotatable member, the two rotatable members being relatively axially displaceable with respect to one another. Helical grooves are formed on an inner cylindrical surface of the first rotatable member and on the outer cylindrical surface of the second rotatable member. Balls that are held in position relative to one another by means of a cage are engaged in the helical grooves of the two members. Adjustment means are provided for bringing about a phase change by causing relative axial displacement of the first and second rotatable members.
The second rotatable member may be formed as an intermediate cylinder or sleeve between an inner rotatable shaft and the first rotatable member, the inner rotatable shaft and the intermediate cylinder or sleeve being coupled for rotation together by a coupling with allows relative axial displacement thereof, or the first rotatable member may be formed as an intermediate sleeve or cylinder between the second rotatable member and an outer rotatable member, the outer rotatable member and the intermediate cylinder being coupled for rotation together by a coupling which allows relative axial displacement thereof.
In EP-A-0723094, the coupling between the intermediate member and one of the inner rotatable shaft or the outer rotatable member, that is to say one of the drive and driven members, is by means of axial grooves which simply allow the intermediate member to move axially without bringing about any relative phase shift. Furthermore, it is essential in this earlier proposal to use cages for each set of balls.